This invention generally relates to refrigeration systems, and is specifically concerned with a method for controlling a transport refrigeration system that prevents undesirable top-freezing of items stored in the conditioned space without modulating the amount of refrigerant flowing through the system heater-evaporator coil.
Methods for controlling transport refrigeration systems in a manner that prevents top-freezing are known in the prior art. An example of such a method is disclosed and claimed in U.S. Pat. No. 5,172,560, likewise assigned to the Thermo King Corporation located in Minneapolis, Minn. The refrigeration system itself generally comprises a refrigerant compressor driven by a diesel engine, at least one evaporator-heater coil, and a fan for discharging a flow of air through the coil to cool or heat a conditioned space to a selected temperature setpoint. Such systems further include a sensor for measuring the temperature of the discharged air from the fan, as well as a sensor for measuring the temperature of air that has circulated through the conditioned space. Such systems may be operated in either a cooling or a heating mode wherein either an expanded or compressed refrigerant is circulated through the evaporator-heater coil. Such systems may be operated in either a high speed or a low speed mode corresponding to either the 2200 rpm or 1400 rpm speed that the diesel engine is conventionally operated.
In prior art methods for operating such refrigeration systems, the system is initially operated in a cooling mode at high speed to effect an initial temperature pull down that rapidly brings the temperature of the conditioned space (as measured by the temperature of the return air) to a control temperature that is somewhat higher than the setpoint. As soon as the control temperature is obtained, the system continues to operate in a high speed cooling mode while a controlled amount of hot gas is injected into the evaporator-heater coil via the hot gas injection valve. The introduction of such hot gas into the coil slows the rate of temperature drop in the conditioned space. As the system achieves a second control temperature even closer to setpoint, it is operated at low speed while hot gas continues to be injected into the coil through the hot gas injection valve, thus further slowing the rate of temperature drop in the conditioned space. Thus temperature overshoot in the conditioned space is prevented, and stability around setpoint is attained by the modulation of the refrigerant entering the evaporator-heater coil via the hot gas injection valve, which acts as an ever increasing thermal "brake" as setpoint is approached. The prevention of such temperature overshoot is highly desirable, as it prevents potentially damaging top-freezing from occurring to the goods stored in the conditioned space.
While such control methods have proven their practicality in the field, the applicants have noted a number limitations of such methods that impair their usefulness. For example, such methods are not readily adaptable for use in refrigeration systems having multiple compartments that are maintained at different temperatures. In such systems, a single refrigerant compressor and diesel engine drives a plurality of evaporator-heater coils and fans to individually cool or heat each of the various spaces, and no special valves or other mechanisms are normally provided for the controlled modulation of hot gas in each of the various evaporator-heater coils. Moreover, even in refrigeration systems having only a single evaporator-heater coil and fan for conditioning only a single space, it would be desirable to have a control method that did not rely upon a hot gas modulation valve since such valves are relatively expensive and complex mechanisms.
Clearly, there is a need for an improved method for controlling a transport refrigeration system that does not rely upon a hot gas modulation valve, but yet which is capable of rapidly pulling a conditioned space down to a selected temperature setpoint without the occurrence of temperature overshoot that can cause undesirable top freezing of the items stored in the conditioned space.